Building on expertise

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Simeon Collins, a director at heat treatment and hard coatings specialist Wallwork, explains some of the new developments since the last Farnborough Airshow that anticipate the needs of aerospace primes.

When it comes to meeting some of the heat treatment and hard coatings challenges, aerospace can be a very demanding industry. Commercial, technical and environmental challenges drive component manufacturers to constantly invest and upgrade facilities, processes and quality systems.

Metallurgical expertise is the core skill in the Wallwork group. This can be applied in thermal processing, specialist hard coatings and research and development to address new technical issues. Our main aim is to alter the surface characteristics of metal components to make them more resilient or lubricious.

We are looking closely at additive manufacturing (AM) as a means of making aerospace components to address demands for strength, lightness and specific surface requirements. Our conclusion for now is that they do not immediately deliver what the market needs. We have therefore invested substantially in expanding our vacuum brazing capacity, since this brings deliverable benefits for the efficient production of complex components that meet the current technical requirements of aerospace.

Bearings

Vacuum brazing is a fully-developed and proven additive technology, offering a flexible route to the production of complex components. It is available now, accredited by leading international standards bodies and is able to meet multiple technical challenges in aerospace, defence and other demanding engineering fields.

Complex components can be fabricated by joining simpler parts using a braze alloy with a lower melt point than the base material and that flows by capillary action into the joint space between the parts. The process produces high strength components with excellent dimensional stability, free of voids or inclusions, resistant to shock and vibration and able to withstand high pressures.

Over the last year we have more than doubled our vacuum brazing capacity. In addition to the extended vacuum brazing laboratory in Cambridge we have mirrored these facilities at Manchester so we can respond quickly and give aerospace manufacturers the greater security of two-site operation. We can assemble anything from micro components to large fabrications. The installation of honeycomb seal rings and segments for aero engine are a typical application where the complex geometry can be bonded to create a perfect seal that is fully verified.

New material challenges

Save weight, save fuel, reduce CO2, extend time on-wing and the mitigation of other environmental impacts continues to be the mantra for aircraft builders and operators. Change, however, is moderated by safety concerns, so each step is planned, subject to testing, quality control and long term monitoring.

Magnesium is a now a common substitute for aluminium. Titanium is increasingly used as a substitute for steel and other dense metals. Carbon fibre is replacing aluminium and in aero engines, new alloys, manufacturing and coating techniques are extending performance. Widely used processes, such as hard chrome plating are becoming illegal and alternatives are required that do not use toxic materials or produce harmful residues.

Bearings and undercarriage components are one field where light and strong titanium is taking over from steel. However, the coefficient of friction of titanium and the propensity for metal to metal adhesion can undermine advantages on heavily-loaded surfaces.

To overcome these limitations, we offer Nitron O duplex coating. The titanium surface is first hardened using a plasma based process to create hard and deep nitrided cases in the alloy. Within the same process cycle either a titanium or chromium based layer is then applied by physical vapour deposition (PVD) creating a surface that has steel-like properties – making it both lubricious and resilient.

Other issues arise in aero engines. High temperature, pressure and erosion from air stream contaminants in the compressor stage, change the geometry of aerofoils, increasing fuel consumption and reducing service life. Nitron Flight, a suite of multi-layer coatings based on titanium, chromium, chromium/aluminium or carbon/metal carbide overcome these problems.

Applied in six to eight micro layers, making a surface that complies closely with the super polished substrate, the low stress surface coating is more resilient to delamination than a single layer coating of the same thickness. The coating improves fuel consumption maintaining the optimum smoothness profile for sustained performance.

Machining carbon composites

Cutting and machining carbon fibre reinforced plastic (CFRP) composites or stacks combining CFRP with aluminium or titanium is hard on tooling. The heat generated can cause tooling to smear the resins, rapid wear can create ragged cuts and sometimes delamination to undermine quality and safe performance.

The answer lies in diamond coated carbide tooling. To provide this service, Wallwork partners with CemeCon, for whom we are the only official UK service provider. We offer a complete suite of industrial diamond coatings for tools used in the machining of composites and stacks.

Cemecon

These coatings have good surface adhesion, outstanding hardness and good thermal conductivity. Staying sharper for longer means fewer tool changes, longer runs and higher productivity in addition to cleaner cutting, higher yields and improved quality.

For decades hard chromium plating (HCP) has been the engineers’ choice for wear surfaces subject to harsh environments, but health and safety concerns and environmental constraints on the process demand an alternative. We advocate Nitron CA a form of chromium nitride (CrN).

Nitron CA is twice as hard as HCP and forms a good bond to the substrate. Deposited as an even layer it follows the contours of the part closely and maintains coverage on edges and corners. It shares many of the characteristics of HCP: it is chemically unreactive and therefore stable and corrosion resistant, the surface is smooth and therefore highly lubricious, and it is heat resistant and can be applied to high thicknesses.

Frontier collaboration

Cambridge is our research and development centre. Here we have a concentration of scientists and engineers, experienced in developing resistive coatings, with connections to major universities and research establishments.

Most of the vacuum process equipment that we use is developed here, so the team are well-equipped to research and refine every aspect of coating and application technology. This resource is available to aerospace majors and others who need customised coatings for special applications.

Our goal, since the last Farnborough Airshow has been to extend the range of services that we offer and deepen the accreditations from global bodies, such as Nadcap and industry primes. In addition to the expanded capacity for vacuum brazing, we have also increased our capability to harden both aluminium and magnesium alloys. The depth of nitriding services has also been extended such that we offer both thermal and gas/plasma services so we can offer clients the best technical solution in every case.

www.wallworkht.com

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Wallwork [**]

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